1.Field of the Invention
The present invention relates to a recording head that ejects ink to perform recording, and more specifically, it relates to a recording head having a mechanism for removing accumulated air that causes defective ink ejection, and a recording apparatus.
2.Description of the Related Art
Ink jet recording apparatuses have an ink jet recording head mounted on a carriage reciprocating above a recording medium. When the recording head reciprocates above a recording medium, an ejection section of the recording head is driven, ink is ejected from ejection ports of the recording head, and an image or characters are formed on the recording medium. The recording head is supplied with ink from an ink supply container (hereinafter referred to as ink tank).
Some recording apparatuses have a detachable ink tank. Ink is supplied from the ink tank to a recording head through a tube.
In general, a recording head is located above a recording medium in the direction of gravitation and downward ejects ink droplets. It is preferable that the inside of the recording head be maintained at a negative pressure slightly lower than the atmospheric pressure, and the ink in each ejection nozzle form a meniscus.
Air in a recording head can cause trouble in an ink jet recording apparatus. Air in a recording head prevents ink from being supplied to the recording head. Supplying air instead of ink to an ejection section results in defective recording. The air in the recording head changes its volume with changing ambient temperature. With changing volume of the air in the recording head, the inside of the recording head maintained at a negative pressure slightly lower than the atmospheric pressure can be put under positive pressure. In the case of a recording head having nozzles facing downward, if the inside of the recording head is put under positive pressure, it becomes difficult to maintain meniscuses, and ink can flow out of the nozzles.
The entry of air into a recording head occurs in the following cases.
When an ink tank is replaced, and when a connector portion of a supply tube connected with the ink tank is temporarily opened to the atmosphere, air enters the supply tube through the connector portion. Specifically, in a configuration in which a supply tube is passed through a rubber plug of an ink tank, the open end of the supply tube is exposed to the atmosphere when the ink tank is replaced. Through the open end, air can enter the supply tube. After a new ink tank is attached, the air that has entered the supply tube is supplied through the tube to a recording head.
Air can also enter a tube constituting an ink path through the wall of the tube over a long time, and the air can be sent together with ink to a recording head and can stay in the recording head. A tube used for supplying ink to a recording head mounted on a carriage that reciprocates, is required to have flexibility and therefore is formed of a resin material having a low gas permeability, such as polyethylene. However, when an apparatus is unused for a long time, a small amount of air passes through the wall of the tube. The air that has entered the tube is introduced into the recording head by recording operation.
Air dissolved in ink can accumulate due to changes in ambient condition. Air dissolved in ink accumulates due to changes in temperature or pressure caused by ejection, and forms bubbles around an ejection section.
Air enters a recording head through nozzles during ink ejection. Air also enters a recording head when the air in the recording head changes its volume with decreasing ambient temperature and a larger-than-normal negative pressure breaks meniscuses.
The accumulated air in a recording head changes its volume with decreasing ambient temperature, and further air is taken in through nozzles.
WO 98/08876 discloses the idea that a recording head has a space for holding air, and the time to replace the recording head is designed by considering the amount of entering air and the time when trouble occurs.
Japanese Patent Laid-Open No. 7-266571 discloses a recording head that is mounted on a carriage and provided with an ink inlet tube, an ink outlet tube, and a pump. By circulating air between the recording head and an ink tank, air bubbles in the recording head is forced to flow into the ink tank.
Japanese Patent Laid-Open No. 2006-159482 discloses a configuration to easily discharge air bubbles in a filter for removing foreign substances disposed in an ink flow passage.
U.S. Pat. No. 7,150,519 discloses the state of air accumulated in a recording head, how to fill the recording head with ink, and how to remove the accumulated air. Ink jet recording apparatuses are required to perform high-definition recording at high speed on large-sized recording paper. Therefore, the frequency of replacing recording heads increases, and the length of a tube that supplies ink from an ink tank to a recording head increases. Consequently, the amount of air entering during replacement of an ink tank, and the amount of air entering an ink flow passage through the wall of the tube, also increase. In addition, increasing the number of nozzles for high-speed recording results in an increase in the amount of air entering through nozzles.
In the case of a configuration in which entering air is held in a recording head as described in WO 98/08876, an increase in the amount of air entering a tube that supplies ink from an ink tank to the recording head and the amount of air entering the recording head through nozzles leads to an increase in the size of the recording head and results in an increase in the size of the apparatus.
In the case of the configuration of WO 98/08876, if the size of the recording head, in which entering air is held, is not increased, the frequency of replacing recording heads increases.
Recent ink jet recording apparatuses have a large recording width and require high-rate ink supply. Some of them need pressurized supply of ink from an ink tank to a recording head. In the case of pressurized supply of ink to a recording head, it has become necessary to provide the recording head with a negative pressure generator and to remove air that has entered the recording head.
In the configuration of Japanese Patent Laid-Open No. 7-266571, ink is returned from a recording head to an ink tank by a pump attached to the recording head, and at the same time, air accumulated in the recording head is sent to the ink tank. Therefore, in the configuration of Japanese Patent Laid-Open No. 7-266571, air is not completely discharged to the outside of the recording head and the supply system.
U.S. Pat. No. 7,150,519 discloses a configuration in which a suction cap is brought into contact with ejection ports of a recording head and air is forcibly sucked out of the recording head. In the configuration of U.S. Pat. No. 7,150,519, ink is sucked together with air, and therefore ink is wasted.